Cholera broke out in Haiti two years ago, and more than 7,000 people have died. Some researchers traced the outbreak's origin to United Nations peacekeepers sent from Nepal after the devastating earthquake in 2010.

The theory that Nepalese soldiers unwittingly spread the bacterial gastrointestinal ailment has become widely accepted based on genetic fingerprints revealing the strain's Asian roots.

Now research from the University of Maryland School of Medicine and College Park campuses is painting a more complicated picture, with recent findings showing that a second cholera strain also sickened some Haitians. The findings indicate that this strain exists naturally in the environment, was awakened from dormancy by unusually warm weather and then spread amid the social upheaval caused by the earthquake.

The research, by a team that included Rita Colwell, a University of Maryland microbiologist, highlights the second strain's significance. Because it was the only pathogen found in 21 percent of the ill Haitians tested for her study, "its role in the epidemic, either alone or in concert with" the first strain attributed to the Nepalese soldiers "cannot be dismissed," she and her co-authors wrote.

The report, published recently in the Proceedings of the National Academy of Sciences, has stirred debate among cholera experts. Critics have noted that the second strain isn't a type linked to earlier epidemics. Others have questioned the paper's climate data. Harvard molecular biologist John Mekalanos told Science magazine he thought the paper "tries to obscure the published facts and invent controversy."

Colwell, who has studied cholera for 40 years and is a former director of the National Science Foundation, said she is only reporting what she found. "We did not challenge those who have claimed it is the Nepal strain," she said in an interview. "We have simply presented our scientific data and concluded it is rather complicated."

To those who definitively attribute the epidemic to the Nepalese, she asks: "Where's the proof?"

The debate has implications beyond academia. Colwell said her findings could help with prevention by offering clues about the environmental conditions that foster the spread of cholera bacteria. There is a political dimension, too: Reports attributing the epidemic to a sewage leak at the Nepalese battalion's riverside camp have sparked protests, echoing a centuries-old narrative.

Haitians are "immensely proud that they were the first slave nation to throw off their shackles and become independent — hugely proud of that," said Tom Price, senior communications manager with Catholic Relief Services, the Baltimore-based organization that has a major presence in Haiti. "The flip side is there is a wariness of foreign involvement, and some of it's justified."

The idea that cholera started with the Nepalese soldiers "feeds into the existing stereotype," he said.

Dr. David Sack, an epidemiologist at the Johns Hopkins University's Bloomberg School of Public Health, defended both Colwell and Mekalanos, saying, "I think they're both excellent scientists and just have different ways of looking at things."

Sack said he has doubts about what he called "the Nepalese hypothesis" despite a U.N. commission's "pretty clear" conclusion that it's correct. "There are parts of the story that are not totally consistent — the timing of the epidemic and the timing of when the soldiers arrived," he said.

But Sack said those doubts do not lead him to conclude that the strain identified by Colwell did cause the cholera outbreak, the first recorded in Haiti in more than a century, according to the U.S. Centers for Disease Control and Prevention.

Sack said the strain linked by some researchers to the Nepalese — referred to as "01 serotype" — clearly came from outside Haiti and spread across the nation. He thinks it remains an open question of how it reached Haiti.

As for Colwell's research, which detected the presence of the "non-01" strain, it's "not contradicting the Nepalese hypothesis at all," Sack said. "What it is showing is that the environmental conditions that favored the spread of cholera favored not just the 01 [type] but some non-01 as well."

Colwell and Sack said cholera, which can cause severe diarrhea, dehydration and death, can be found in nature in many places, probably including the Chesapeake Bay.

"But we don't worry about it because it's not allowed to spread," Sack said. "In Haiti, the conditions are such — sanitation, the water supply is so horrible — that it facilitates the spread."

Price said that to many Haitians, it's established fact that the Nepalese brought cholera. Even if scientific evidence discredits that view, he said, "it really doesn't matter. That is a reality, and it has caused quite a bit of trouble and bad feeling."

CRS, meanwhile, has continued to work closely with the Haitian Catholic hospital network to treat those with cholera and raise awareness about the importance of clean water and sanitation.